The present invention relates generally to an apparatus and method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and more particularly, to an apparatus and method for increasing the lateral measurement of nuclear magnetic resonance properties of an earth formation.
It is well recognized that most of the particles of an earth formation having non-zero nuclear spin magnetic moment, for example protons, have a tendency to align with a static magnetic field imposed on the formation. Such a magnetic field may be naturally generated, as is the case for the earth's magnetic field, B.sub.E. After an RF pulse is supplied by a second oscillating magnetic field B.sub.1, transverse to B.sub.E, the protons will tend to precess about the B.sub.E vector with a characteristic resonance or Larmor frequency .omega..sub.L which depends on the strength of the static magnetic field and the gyromagnetic ratio of the particle. Hydrogen nuclei (protons) precessing about a magnetic field B.sub.E of 0.5 gauss, for example, have a characteristic frequency of approximately 2 kHz. If a population of hydrogen nuclei were made to precess in phase, the combined magnetic fields of the protons can generate a detectable oscillating voltage, known to those skilled in the art as a spin echo, in a receiver coil. Hydrogen nuclei of water and hydrocarbons occurring in rock pores produce NMR signals distinct from signals arising from other solids.
The NML.TM. nuclear magnetic logging tool of Schlumberger measures the free precession of proton nuclear magnetic moments in the earth's magnetic field. See U.S. Pat. No. 4,035,718 issued to Richard N. Chandler. The tool has at least one multi-turn coil wound on a core of non-magnetic material. The coil is coupled to the electronic circuitry of the tool and cooperatively arranged for periodically applying a strong DC polarizing magnetic field, B.sub.p, to the formation in order to align proton spins approximately perpendicular to the earth's field, B.sub.E. The characteristic time constant for the exponential buildup of this spin polarization is called the spin-lattice relaxation time, T.sub.1. At the end of polarization, the field is rapidly terminated. Since the spins are unable to follow this sudden change, they are left aligned perpendicular to B.sub.E and therefore precess about the earth's field at the Larmor frequency f.sub.L =.gamma.B.sub.E, where .gamma. is the gyromagnetic ratio of the proton. The Larmor frequency in the earth's field varies from approximately 1300 to 2600 Hz, depending on location. The spin precession induces in the coil a sinusoidal signal of frequency f.sub.L whose amplitude is proportional to the number of protons present in the formation. Additives in the borehole fluid are required to prevent the borehole fluid signal from obscuring the formation signal. The tool determines the amount of free fluid in the formation, the remainder of the pore space assumed to be occupied by bound fluid.
A further nuclear magnetic resonance approach employs a locally generated static magnetic field, B.sub.o, which may be produced by one or more permanent magnets, and RF antennas to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, and permeability of a formation. See U.S. Pat. No. 4,717,878 issued to Taicher et al. and U.S. Pat. No. 5,055,787 issued to Kleinberg et al. Nuclear spins align with the applied field B.sub.o with a time constant of T.sub.1 generating a nuclear magnetic moment. The angle between the nuclear magnetization and the applied field can be changed by applying an RF field, B.sub.1, perpendicular to the static field B.sub.o. The frequency of the RF field must be the Larmor frequency. After application of an RF pulse, the protons begin to precess in the plane perpendicular to B.sub.0 and generate a sequence of spin-echoes which produce a detectable signal in the antenna.
Nuclear magnetic resonance has proven useful in medical applications to perform noninvasive examinations of the interior organs and structures of an organism. See P. Mansfield, Pulsed Magnetic Resonance: NMR, ESR, and Optics, 317-345 (D. M. S. Baugguley ed., Cleardon Press, Oxford, 1992). The desire for faster imaging led to the development of commercial and laboratory NMR imaging systems in the medical field which use various gradient-echo techniques consisting of radiofrequency pulses, .alpha., in combination with switched magnetic field gradients to generate an image. See Stewart C. Bushong, Magnetic Resonance Imaging: Physical and Biological Principles, 279-286, (2d edition 1996). Known techniques such as fast low angle shot (FLASH) and fast imaging with steady state precession (FISP) require an RF excitation pulse, .alpha., of approximately 90.degree. while other techniques vary the flip angle between 30.degree. and 70.degree. to maximize magnetic resonance strength.
While the tools and techniques developed in the prior art extract information about fluid properties, the tools and techniques have a disadvantage which limit their utility in practical applications. With a nuclear magnetic logging tool, as explained in U.S. Pat. No. 4,717,877 issued to Taicher et al., shell regions of differing radial separations from the longitudinal axis may be subjected to nuclear magnetic resonance excitation by varying the RF field frequency. Due to the required application of the RF field, the precession frequency is fixed, and the measurement, lateral from the borehole axis, of nuclear magnetic resonance properties of an earth formation is constrained to a thin shell region which provides a shallow depth of investigation relatively close to the borehole wall. Therefore, there is a need for a nuclear magnetic resonance system and method for determining a characteristic of an earth formation which does not require the use of an RF pulse to generate spin echoes.